Highly Efficient Degradation of Emerging Contaminants with Sodium Bicarbonate-Enhanced Mn(II)/Peracetic Acid Process: Formation and Contribution of Mn(V)

Abstract

Organic ligands have been extensively used to enhance the catalytic performance of manganese ion (Mn(II)) for peracetic acid (PAA). In this study, sodium bicarbonate (NaHCO₃), an economical and eco-friendly inorganic ligand, was introduced to enhance the degradation of emerging contaminants (ECs) in the Mn(II)/PAA process. NaHCO₃ could significantly improve the oxidizing ability of the Mn(II)/PAA process over the initial pH range of 3.0–11.0. Mn(V) was identified as the primary reactive species for degrading naproxen in the NaHCO₃/Mn(II)/PAA process. HCO₃^– could complex with Mn(II) to generate Mn(II)-HCO₃^–, which has a lower redox potential to enhance the catalytic activity of Mn(II). Mn(II)-HCO₃^– reacted with PAA to produce Mn(III)-HCO₃^– and CH₃C(O)O^•. Mn(V)-HCO₃^– was generated via two-electron transfer between Mn(III)-HCO₃^– and PAA. Although organic radicals were detected in the NaHCO₃/Mn(II)/PAA process, naproxen was mainly degraded by Mn(V)-HCO₃^– via one-electron transfer along with the formation of MnO₂. Notably, the coexisting hydrogen peroxide was vital in the reduction of MnO₂ to Mn(II/III), thereby enhancing the continuous generation of Mn(V)-HCO₃^–. NaHCO₃/Mn(II)/PAA process exhibited exceptional oxidation performance in actual water samples. This study proposed a strategy utilizing an eco-friendly inorganic ligand to address the inherent drawbacks of organic ligand-enhanced Mn(II)/PAA processes and highlighted its potential applications in the removal of ECs.